Mirrors used to reflect electromagnetic radiation are typically comprised of a reflective coating applied to a substrate which has been polished to a low roughness and a well defined surface figure. In some cases, however, it is difficult to achieve the required substrate surface conditions prior to the application of the reflective coating. The substrate may inherently have poor polishing characteristics or it simply may not be polishable to the level required to produce the necessary mirror properties. Even if the substrate is polishable to the required surface texture, the substrate material may be chemically or thermomechanically incompatible with the reflective coating which is applied to it, which complicates the fabrication of certain types of mirrors.
One technique that has been used in an attempt to alleviate such problems has been to apply an intermediate (buffer) layer between the substrate and the reflective layer. This intermediate layer is readily polishable, and provides the necessary transition characteristics between the substrate and the reflective layer. Mirrors having multiple coating layers (i.e., at least one intermediate layer and at least one reflective layer) are discussed in, for example, U.S. Pat. Nos. 4,101,707 to Henry, U.S. Pat. No. 4,142,006 to Choyke et al, 4,444,467 to Shuskus et al, and U.S. Pat. No. 4,451,119 to Meyers et al. These multiple coating layers are typically applied by techniques such as chemical vapor deposition (CVD), physical vapor deposition (PVD), or other material transport processes such as ion vapor deposition (IVD) and sputtering. While these processes are generally capable of depositing high quality coating layers, their deposition rates are relatively low and they are therefore costly to use. Depending on the size and shape of the substrate, and the coating composition which is applied thereto, it can take several hours to deposit the mirror coating to the required thickness. As mirror substrates become larger and more complex in shape, the time required to deposit and then polish the mirror coatings invariably increases.
Accordingly, what is needed is a more cost effective method for applying coatings to mirror substrates. These coatings should also be easy to polish. If these coating systems could be developed, such an improvement in the state of the art would reduce the time and cost of fabricating certain types of mirrors. It might also enable the fabrication of other mirrors which to date have not been fabricable.